Field of the Invention
The present invention relates to a light deflection unit including a rotary member having a rotor frame, an optical scanning apparatus, and an image forming apparatus.
Description of the Related Art
In general, an optical scanning apparatus to be used for an image forming apparatus such as a laser printer optically modulates a laser beam emitted from a light source in accordance with an image signal, and deflects and scans the optically-modulated laser beam with a light deflection unit including a rotary polygon mirror. The deflected and scanned laser beam is optically scanned by a scanning lens such as an optical imaging system having an fθ characteristic under a state of forming an image in a spot-like manner on an image bearing member such as a photosensitive drum, to thereby form an image.
In order to reduce dynamic imbalance of the rotary member, which is caused by rotation, a weight for balance correction is provided to a rotor frame or the rotary polygon mirror of the light deflection unit. When initial imbalance (state before the balance correction) is large, the amount of addition of the balance weight increases to increase a height of the balance weight in some cases. When the rotary member provided with the balance weight as described above rotates, air strikes on the balance weight to generate fluid dynamic noise. In Japanese Patent Application Laid-Open No. 2006-058723, the problem of generation of the fluid dynamic noise is reduced by defining the height of the balance weight.
In the related-art example described above, however, the balance weights are provided to a plurality of portions in some cases. Therefore, the related-art example has the following problem. Specifically, the fluid dynamic noise generated by the balance weight is noise having a frequency one to three times as high as a motor rotation period (noise having a frequency twice as high as the motor rotation period is particularly noticeable). In recent years, the fluid dynamic noise generated in the light deflection unit that is rotationally driven at a high speed becomes extremely annoying. The fluid dynamic noise is generated for each balance weight. Therefore, when the balance weights are provided to a plurality of portions, the fluid dynamic noise is generated at each of the plurality of portions where the balance weights are respectively provided. For example, in a case where the balance weights are excessively added in a first balance correction and the amount of imbalance of the rotary member of the light deflection unit fails to fall within a defined amount, a second balance correction is implemented. In this case, a position where the second balance correction is implemented is in an opposite phase to that of the position of the first balance correction (is a position rotated at 180°). If the rotary member of the light deflection unit rotates under this state, the fluid dynamic noise is generated at the two positions where the balance weights are respectively provided. As a result, the fluid dynamic noise becomes larger than that in the case where the balance weight is provided to one position.